The radiation detectors used in X-ray tomography devices serve to convert X-radiation into electrical signals, which form the starting point for image reconstruction. Currently, indirectly converting radiation detectors based on scintillators are used. With this type of detector, the conversion of the X-rays takes place in two stages. In a first stage, the X-rays are absorbed by means of a scintillator and converted into optically visible light pulses. The scintillator is structured into pixels in order to achieve a certain position resolution. For example, the following compounds are suitable as a scintillator material: Gd2O2S:Pr/Ce, (Y,Gd)2O3:Eu, (LuxTby)3Al5O12:Ce, CsI:Tl, CsI:Na and CdWO4. The generated light pulses are subsequently converted into electrical signals in a second stage by a photodiode array optically coupled to the scintillator. The photodiode array is made up of a multiplicity of individual photodiodes, and has a structure corresponding to the scintillator.
The known radiation detectors in the field of human medicine operate in an integrative mode, which integrates over the signals of all the radiation quanta which arrive at a pixel within a certain time window. The recorded signals are essentially proportional to the sum of the energy values of the individual incident radiation quanta. However, the information about the number and energy of the individual radiation quanta is lost.
Quantitative and energy-selective detection of the radiation quanta, however, offers a number of advantages specifically in the field of human medical imaging. With quantitative detection of the radiation quanta, for example, image generation is possible with a comparatively low X-ray dose. Additional energy-selective detection of the radiation absorbtion furthermore gives the possibility of material-specific representation and evaluation of the image information.
Such detection proves to be difficult in the field of human medicine, however, since in this case quantum absorption events with comparatively high quantum fluxes, for example more than 108 X-ray quanta/mm2*s, have to be counted.
In order to produce counting radiation detectors for such high quantum fluxes, in a first approach so-called directly converting radiation detectors are studied. In this type of detector, an incident radiation quantum generates free charge carriers in the form of electron-hole pairs, which are also referred to as excitons, in a converter layer as a result of sometimes multistage physical interaction processes with a semiconductor material. For example, semiconductor compounds such as CdTe, CdZnTe, CdTeSe or CdZnTeSe are studied as materials for the converting layer. These materials have a high X-ray absorption in the energy range of medical imaging. A hitherto unresolved problem, however, is that the production process gives rise to defects, owing to which the liberated charge carriers are demobilized, that is to say slowed down or trapped. This so-called polarization effect reduces the separation efficiency of the liberated charge carriers, and leads to broadening of the detected electrical signal. This entails the risk that signals of quanta arriving in close succession will be superimposed so that it is not possible to separate the events. For this reason, directly converting radiation detectors have to date been usable only very limitedly for counting detection of absorption events with high quantum fluxes.
Another approach for the production of a counting detector for high quantum fluxes consists in producing an indirectly converting radiation detector with a faster scintillator and a faster photodiode array. U.S. Pat. No. 7,403,589 B1 discloses such an optically counting radiation detector, in which silicon photomultipliers (SiPM) are used to convert the light pulses generated by the incident radiation quanta into electrical signals. These are special avalanche diodes, which are operated in Geiger mode. In this type of operation, a bias voltage having a value which is close to the breakdown voltage is applied to the diodes. A single exciton can therefore trigger breakdown with a high current value within a few nanoseconds. With this technology, it is possible to achieve low-noise detection of the scintillation photons with a high temporal and spatial resolution which is necessary for high-flux application. A limiting factor of counting detection is currently the comparatively slow decay time of the scintillator. The scintillator known from U.S. Pat. No. 7,403,589 B1 comprises a scintillator material from the following group: LYSO, LaBr3 or LuTAG. These scintillators have decay times in a range of from 15 ns to 50 ns. For counting detection of absorption properties with high quantum fluxes of more than 108 X-ray quanta/mm2*s, however, scintillators which have decay times of less than 5 ns are necessary.